An alkaline storage battery using a hydrogen-absorbing alloy as the negative electrode has excellent safety and is therefore used for high power applications such as HEVs and PEVs.
The hydrogen-absorbing alloy is commonly composed of a single-phase of an AB2 type structure or AB5 type structure. However, recently, the hydrogen-absorbing alloy has been required to have much higher power or much higher capacity performance than the conventional range. Accordingly, a hydrogen-absorbing alloy including as the main phase an A2B7 type structure or A5B19 type structure in which an AB2 type structure and AB5 type structure are combined, such as a rare earth-Mg—Ni-based alloy has been proposed. (International Publication WO 2007/018292)
The crystal structure of the rare earth-Mg—Ni-based hydrogen-absorbing alloy is transformed based on its stoichiometric ratio. That is, when the stoichiometric ratio is increased, the A5B19 type structure becomes dominant from the A2B7 type structure.
Because the A5B19 type structure has a periodically stacked structure including one layer of the AB2 type structure and three layers of the AB5 type structure, the nickel ratio per unit crystal lattice can be improved, and therefore, an alkaline storage battery using the rare earth-Mg—Ni-based hydrogen-absorbing alloy that contains (a relatively large amount of) the A5B19 type structure as the main phase shows especially excellent high power.
On the other hand, the high power application for HEVs commonly employs a partial charge-discharge control system in which pulse charge and discharge are repeated, for example, in the range of a state of charge (SOC) from 20 to 80%.
Accordingly, in the high power application for HEVs, the alkaline storage battery to be used is required to have excellent power characteristics as well as power characteristics with small variation associated with SOC variation (excellent output power stability).
Generally, the power characteristics of the alkaline storage battery containing the hydrogen-absorbing alloy closely relates to the absorption hydrogen equilibrium pressure of the hydrogen-absorbing alloy. That is, when the hydrogen-absorbing alloy has a high absorption hydrogen equilibrium pressure, the power characteristics become high, and when the hydrogen-absorbing alloy has a low absorption hydrogen equilibrium pressure, the power characteristics become low.
Consequently, when the absorption hydrogen equilibrium pressure of the hydrogen-absorbing alloy varies associated with the SOC variation, the power characteristics vary.
When the power characteristics vary associated with the SOC variation, a predetermined output power cannot be obtained in a certain SOC range. Thus, the variation of the power characteristics associated with the SOC variation is not preferable for the high power application for HEVs that requires a constant output power over from low SOC to high SOC.
Therefore, in order to reduce the variation of the power characteristics associated with the SOC variation, it is necessary to control the hydrogen-absorbing alloy so that the absorption hydrogen equilibrium pressure varies in a small range associated with the SOC variation. That is, it is necessary to control the hydrogen-absorbing alloy so that the variation of the absorption hydrogen equilibrium pressure is reduced in a plateau region of a PCT curve of the hydrogen-absorbing alloy (a region observed in the range of an SOC of 20 to 80%, where the absorption hydrogen equilibrium pressure of the hydrogen-absorbing alloy does not largely vary associated with the SOC variation) corresponding to a practical region.
In particular, when a rare earth-Mg—Ni-based hydrogen-absorbing alloy having the A5B19 type structure (because the crystal structure of the hydrogen-absorbing alloy has poor stability, subphases such as an A2B7 type structure, AB5 type structure, and AB3 type structure are readily generated) as the main phase is used in order to obtain high power characteristics, the alloy has the problem that such subphases reduce the flatness in the plateau region of the PCT curve of the hydrogen-absorbing alloy to reduce the output power stability. Therefore, when the hydrogen-absorbing alloy is used, it should be noted that the alloy is controlled so that the variation of the absorption hydrogen equilibrium pressure in the plateau region of the PCT curve would be reduced.
Meanwhile, the reason why the subphases reduce the flatness in the plateau region of the PCT curve of the hydrogen-absorbing alloy as discussed above is considered as follows.
Generally, when the hydrogen-absorbing alloy is composed of a plurality of crystal structures, the PCT curve of the hydrogen-absorbing alloy is a mixture (see FIG. 2B) of the PCT curve of each crystal structure (see FIG. 2A).
However, the PCT curves are not equally mixed in all SOC regions, and mixed differently between in a low SOC region and a middle to high SOC region, and thus the finally obtained PCT curve has a tilted plateau region (see FIG. 2B).
This is because, in a low SOC region, a crystal structure having a low absorption hydrogen equilibrium pressure dominantly relates to hydrogen absorption and desorption and, on the other hand, in middle to high SOC regions, a crystal structure having a high absorption hydrogen equilibrium pressure dominantly relates to the hydrogen absorption and desorption. Thus, it is considered that the PCT curves of the hydrogen-absorbing alloy are mixed in the low SOC region so as to shift to the PCT curve of the crystal structure having a low absorption hydrogen equilibrium pressure and, on the other hand, the PCT curves of the hydrogen-absorbing alloy are mixed in a high SOC region so as to shift to the PCT curve of the crystal structure having a high absorption hydrogen equilibrium pressure.
The PCT curve of each crystal structure is mixed as described above and, as a result, the plateau region of the PCT curve of the hydrogen-absorbing alloy is tilted to have poor flatness.
Therefore, it is considered that a battery using such a hydrogen-absorbing alloy has a large variation in the power characteristics associated with the SOC variation to reduce the stability of the power characteristics.
Actually, in the case of the rare earth-Mg—Ni-based hydrogen-absorbing alloy having the A5B19 type structure as the main phase, because the A5B19 type structure as the main phase has a large absorption hydrogen equilibrium pressure and the A2B7 type structure and the like generated as the subphases have low absorption hydrogen equilibrium pressures, when the proportion of the A2B7 type structure and the like as the subphases having low absorption hydrogen equilibrium pressures becomes larger, the PCT curve of the hydrogen-absorbing alloy in the low SOC region is mixed so as to shift to the PCT curve of the A2B7 type structure and the like as the subphase.
As described above, in the rare earth-Mg—Ni-based alloy having the A5B19 type structure as the main phase, it is important to control the structure ratio of the subphases such as the AB3 type structure, AB5 type structure, and A2B7 type structure.
However, the structure ratio control of the subphases by the alloy composition control in related arts cannot sufficiently inhibit the stability reduction of the output power.
Thus, by focusing on a multiphase technique in which manufacturing process control is combined with the alloy composition control, an advantage of some aspects of the invention is to provide a hydrogen-absorbing alloy for an alkaline storage battery having high power characteristics and excellent output power stability and a method for manufacturing the same.